The present invention relates to magnetic disk recording apparatus in general and in particular to an improved magnetic head carriage for use in such equipment.
All magnetic disk recording apparatus to which the present invention is related comprise a circular rotatable disk which is provided with one or more recording surfaces. The disk may be relatively rigid, in which event, the machine in which it is used is called a rigid disk machine, or it may be thin and relatively flexible, in which event, the machine in which it is used is called a flexible disk machine. Due to the flexibility of the latter type of disk, the flexible disks are also known as "floppy" disks because, when stationary, the flexible disk assumes a floppy geometry about its supporting spindle and, when in use, is caused to conform somewhat to the contour of a recording head.
All disk machines use some type of recording head. In rigid disk machines, the head is typically spring-loaded and positioned to "fly" on a film of air created by the rotation of the disk. That is, the head is not in direct contact with the disk surface. By maintaining the head spaced from the rotating disk, the head remains free from damage due to abrasion and like effects which would be likely to occur if the head were held in contact with the disk surface. In contrast, the head in a flexible disk machine is typically held in contact with the disk surface by means of a pressure pad located opposite the head on the opposite surface of the disk.
The recordings on both rigid and flexible disks are made in annular concentric rings or tracks about the axis of rotation of the disk. Moving a recording head from one track to another is called indexing. For indexing a head radially over the surface of a disk there is provided in both flexible as well as rigid disk machines, an assembly called a head carriage. In a rigid disk machine, the head carriage typically comprises a head assembly receiving platform on which a magnetic head assembly is mounted. The platform is provided with rollers or sliders and moved along fixed tracks for moving the head assembly back and forth radially over the surface of the disk by means of an assembly of wires or chains or a combination of these. While capable of precise head-to-track registration, the platform and head assemblies of the rigid disk machines are typically quite massive and expensive.
In flexible disk machines, the head carriage on which a magnetic head assembly is mounted is generally driven by a lead screw. The lead screw is powered by a stepper motor and threadably coupled to the platform for moving the platform and the head assembly mounted thereon radially over the surface of the disk. To compensate for backlash between the lead screw and the platform, it has been the practice to bias the carriage in the flexible disk machines against the threads of the lead screw by means of a spring and a spring-retaining nut threaded on the lead screw. This arrangement, however, has several disadvantages, one of which is due to the effects of cumulative wear which occurs in time between the lead screw threads and the platform. One serious effect of this wear is an ever increasing shift of the platform and head from an initial predetermined position on the disk to a new position in the direction of the spring force against the platform. It can be appreciated that such shifting of the platform and head will in time, unless in some way compensated for, result in interference between the recording and reproduction of recorded signals on adjacent tracks.
In addition to the problem of lead screw thread wear, prior known flexible disk machines using lead screw driven head carriages have also exhibited head skewing caused by erratic movement of the carriage in a direction transverse to the longitudinal axis of the lead screw. This movement is generally attributed to clearances provided between various carriage guiding members on the platform and the lead screw, which are provided of necessity to prevent binding between the lead screw and the platform. In known flexible disk machines, such guiding members may, for example, take the form of a smooth bore in one leg of the platform through which the lead screw is passed for threadable engagement with a second leg.
Heretofore, the effect of thread wear between the lead screw and the platform and the effect of traverse movement of the carriage relative to the longitudinal axis of the lead screw has been compensated for, at least in part, by the employment of a straddle erase head, also known as a tunnel erase head. The straddle erase head comprises a plurality of pole pieces which are arranged in a single head structure for forming a read/write head and a pair of spaced apart erase heads. The erase heads are positioned to overlap the track of the read/write head. Being so positioned, the outer boundaries or peripheral edges of the track of recorded signals made by the read/write head are erased by the spaced erase heads. In this fashion, there is provided, in addition to a narrow zone of unrecorded space between adjacent tracks due to normal indexing of the head carriage, a zone of unrecorded space adjacent both edges of the recorded space in each track. By means of this additional unrecorded zone, destructive interference between recorded signals on adjacent tracks is greatly reduced. As a consequence, the employment of straddle erase heads to compensate for thread wear and unpredictable carriage movement results in an undesirable reduction of valuable recording space and increases the costs of manufacturing. On the other hand, lead screw driven head carriages are otherwise typically simpler, lighter, and less expensive to manufacture and maintain and are, therefore, more desirable than the prior known head carriages used in known rigid disk machines.